This invention relates to a means to prevent fretting, and more specifically, to an anti-fretting washer or shim to be received between two flanges that are clamped together.
Many connections, particularly in power trains, involve the clamping of two flanged parts together. One of many possible examples includes the connection between the output shaft of an aircraft power plant and the hub of a propeller.
In operation of such devices, varying loading on one or the other of the flanged parts causes minute relative movement between the two. This in turn leads to fretting and deformation of one or the other or both of the mounting flanges. Damage to the parts may also occur when the fretting, though not severe enough to physically damage the flanges themselves, causes undue wear of protective coatings or the like, thereby exposing the joint flanges to corrosion or other deleterious effects of the environment.
Needless to say, regardless of the type of equipment involved or the operating environment concerned, such damage is highly undesirable from the standpoint that it must be corrected to assure that the equipment employing the joint operates properly. Moreover, it would be highly desirable that some sort of solution to the fretting problem could be devised. It would be even more desirable that the solution be a long lived solution so as to minimize the need for periodic replacement of components as part of a preventive maintenance program.
The present invention is directed to providing just such a solution.
It is a principal object of the invention to provide a new and improved means of minimizing or eliminating fretting between two mating flanges as may be used in a power train. Even more specifically, it is an object of the invention to provide an anti-fretting shim for use in an aircraft power train as, for example, at the joint between an aircraft power plant output shaft and the hub of a propeller.
An exemplary embodiment of the invention achieves the foregoing object in a shim or washer construction that is adapted for disposition between two mating flanges which are intended to be clamped together. The shim, according to a highly preferred embodiment, comprises a thin, generally circular, planar disc having an open center and an outer peripheral edge. A plurality of angularly spaced bolt holes are located between the open center and the peripheral edge. The shim is a laminate of two sheets bonded together, each sheet consisting essentially of a glass fiber and polytetrafluoroethylene matrix with one side being rich in polytetrafluoroethylene and the other side being rich in glass fiber. The sides of the sheets that are rich in glass fiber are abutted and bonded together.
According to a more specific environment of usage, the invention contemplates use in an aircraft having a propeller including a hub and a power plant for rotating the propeller by a power plant output shaft affixed to the hub. The assembly includes first and second mating flanges and clamping devices for clamping the flanges together. The invention contemplates the improvement of a shim or washer clamped between the flanges including a composite disc having opposed sides and a fiber matrix including a polymeric material with a low coefficient of friction. The distribution of the polymeric material is such that the polymeric material exists in the greatest percentage at the opposed sides and the distribution of the fiber is such that it exists in the greater percentage inwardly of the opposed sides.
In one embodiment, one of the flanges is on the aircraft propeller hub and the other is on the power plant output shaft.
In a preferred embodiment, the clamping device is comprised of bolts.
In another embodiment, the fiber matrix is a fiberglass containing matrix.
Preferably, the matrix is a woven matrix.
In one embodiment, the polymeric material is polytetrafluoroethylene.
In a highly preferred embodiment, the shim is formed of two sheets, each having one side rich in the fiber matrix and an opposite side rich in the polymeric material with the one side of both layers bonded to each other, preferably by an epoxy material.
Other objects and advantages will become apparent from the following specification taken in connection with the accompanying drawings.